A number of prior art communication protocols have been developed to allow an external controller to communicate with a PABX. According to such protocols, the external controller (i.e. host computer) can Obtain infOrmation from the PABX (e.g. the status of predetermined lines and trunk circuits, etc.), to analyze the PABX data base (e.g. in order to determine what features have been programmed into the PABX), and to allow external control of the machine (e.g. in order to place a call, etc.).